docs: document GET /auth/setup in OpenAPI spec

[LumenPrima]

Summary

• Adds GET /auth/setup operation to openapi.yaml as checkAuthSetup
• Response schema matches SetupHandler.Status() exactly: {needs_setup: boolean, user_count: integer}
• security: [] (unauthenticated), tags: [auth], 200 + 500 responses
• Enables dashboard type generation for the first-run setup check (checkNeedsSetup())

Test plan

• Confirm GET /auth/setup is now documented in openapi.yaml
• Verify generated dashboard types cover the checkNeedsSetup() call

Closes AUG-31

🤖 Generated with Claude Code

[LumenPrima]

Closing as a duplicate of PR #45. Multiple agents opened equivalent PRs for documenting GET /auth/setup in openapi.yaml; #45 is the surviving review target.